This seems like a pretty GPU intensive way of getting the effect of just some animated squares. With some ingenuity I think you could do away with the Pixellate effect entirely and take a different approach.
What about using a simple looping, repeatable animated sprite of changing squares. You could do various programmatic things to make it seem more 'random' (picking random frames every so often, or periodically flipping / rotating the sprites...)
Or a tiled background that (while it can't be animated as such) could be given the illusion of 'animation' with some event based methods similar to that which I mentioned above.
This would make it much easier to have the flowing lava follow the screen properly and also make your game much more performant on slower machines.